Polyamide and polyimide coating materials and coatings produced therefrom are generally well known in the art. See, for example, U.S. Pat. No. 3,652,500, issued Mar. 28, 1972, to M. A. Peterson, for "Process For Producing Polyamide Coating Materials By End Capping"; U.S. Pat. No. 3,663,510, issued May 16, 1972, to M. A. Peterson, for "Process For Producing Polyamide Coating Materials"; U.S. Pat. No. 3,507,765, issued Apr. 21, 1970, to F. F. Holub and M. A. Peterson, for "Method For Electrocoating A Polyamide Acid"; U.S. Pat. No. 3,179,614, issued Apr. 20, 1965, to W. M. Edwards, for "Polyamide Acids, Compositions Thereof, And Process For Their Preparation"; U.S. Pat. No. 3,179,634, issued Apr. 20, 1965, to W. M. Edwards, for "Aromatic Polyimides And The Process For Preparing Them"; and U.S. Pat. No. 3,190,856, issued June 22, 1965 to E. Lavin, et al, for "Polyamides From Benzophenonetetracarboxylic Acids And A Primary Diamine". The prior art involves generally the preparation of a coating medium containing a high molecular weight polyamide acid, and application of the coating medium to a substrate to provide a polyamide acid coating thereon, followed by the curing of the high molecular weight polyamide acid to a polyimide. To the extent necessary for a more complete understanding of the present invention, reference should be made to the above listed patents, the disclosures of which are herein incorporated by reference.
Commercially available coating materials for use in electrical applications, such as the coating materials disclosed in U.S. Pat. No. 2,936,296, issued May 10, 1960 to F. M. Precopio and P. W. Fox for "Polyesters From Terephthalic Acid, Ethylene Glycol and a Higher Polyfunctional Alcohol", and used and sold commercially under the trademark "ALKANEX" by General Electric Company, are widely used, highly successful and effective compositions, but are believed to have one economic disadvantage in that they involve and require the use of organic solvents. Where organic solvents are used, they are driven off during cure and are generally not economically recoverable. It is therefore ecologically and environmentally desirable to utilize substantially water based solvents.
Aqueous base polyamide acid systems of the type described in the above-mentioned patents to Peterson result in high temperature electrical grade coatings (130.degree. C., 20,000 hr., class B insulation coating) are stable, and easily made and used, but are believed to be quite expensive compared to the polyester compositions. Aqueous base acrylic systems, of the type described in U.S. Pat. No. 2,787,603, issued Apr. 2, 1957 to P. F. Sanders for "Aqueous Coating Compositions and Substrates Coated Therewith", while inexpensive, are believed to be not generally suitable for high temperature electrical grade coatings applications such as class B applications. Moreover, such aqueous base acrylic systems are emulsions and not solutions, thereby creating certain stability problems.
In water base systems, because of the high latent heat of varporization, it is desirable to utilize as high a solids content as is believed to be possible commensurate with workable viscosities because the medium must be used with automatic coating apparatus such as wire towers. High molecular weight polymers, such as the polyamide polymers which are described in the patents listed above, produce extremely viscous solutions except in relatively low solids content systems. For many applications the low solids content systems are quite suitable. For wire tower use, however, the low solids content aqueous solution is believed to create production problems which reduce the efficiency of the tower and in some cases result in under-cured coatings.